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Re: My preamp coming out...



Luc, (if I understand your post correctly)

1. An S-meter change is not the appropriate measure of the effectiveness of 
a preamp.

2. The fact that your S-meter went full scale both at the antenna and at the 
radio means nothing. Absolutely nothing (other than it has gain).

3. The preamp was designed to have a low noise figure (let's say < 1 dB). 
When you mount it at the antenna you get the benefit of that low noise 
figure. When you mount it at the radio, you add the loss of the coax to the 
noise figure of the preamp. Typically, you might see 2.5 dB loss in your 
feedline at 70 cm. for normal lengths of coax (80'). Now this nice fancy low 
noise preamp has a noise figure of 3.5 dB (which stinks), due to the 
additional loss of 2.5 dB from the coax in front of it.

Unless you have VERY low loss in your feedline run, the preamp belongs at 
the antenna.

For a proper analysis of this issue, it would benefit everyone to look at 
Gene's satellite link budget analysis spreadsheet, which evaluates the 
effects of antenna gain, sky noise, preamp noise figure and gain, feedline 
losses, receive bandwidth and front end noise figure of the radio. Just a 
few minutes manipulating the values in the spreadsheet and seeing their 
effect on receive sensitivity will demonstrate convincingly WHY a preamp 
belongs at the feedpoint, and WHY preamp gain is of only minimal importance 
compared to noise figure.

Noise figure (and overall system noise temperature) is the ONE variable that 
absolutely must be paid attention to, when it comes to weak signal work. 
Assessments based on S-meter readings are worthless and worse, often 
misleading the casual satellite operator.

If a person has any interest at all in learning about how your satellite 
receive system works, and how to optimize it, Gene's spreadsheet (Excel), is 
an indispensible and informative tool.

73,

....hasan, N0AN


----- Original Message ----- 
From: "Luc Leblanc VE2DWE" <lucleblanc6@videotron.ca>
To: <amsat-bb@amsat.org>
Sent: Wednesday, March 22, 2006 1:02 PM
Subject: [amsat-bb] My preamp coming out...


> Yes i make my coming out...
>
> I was not a preamp fan. I was not beleiving too much in preamps virtue but 
> i should now getting out
> the closet in admitting that i try a hamtronic LNG 432 on a SO-50 near 
> overhead pass.
>
> Results:
>
> From a RX signal who was never indicated on the FT-847 meter i was able to 
> get a S9 plus 60db on
> the same meter!! (still tryng to figure out the problem..) I quickly 
> installed the preamp at the
> transceiver not even at the antenna and i got this result.?
>
> Many folks always recommend to install the preamp at the antenna feed 
> point to acheive the best NF
> figures but when you got a full led scale i am just asking why should i 
> put the preamp there?  to
> be battered by the WX ?
>
> With the official spring start yesterday it will be now possible to make 
> further test but it is
> still hard to believe how much amplification this little box can do. I 
> have also an other one in
> standby on VHF just awaiting to be tested too.
>
> For those who are thinking that my FT-847 is probably deaf or dead i just 
> checked it with a cushman
> signal generator and he is better than the Yaesu specs.
>
> Too bad for suit sat it will be an excellent source of weak signals.
>
> A former preamps unbeleiver...
>
>
> "-"
> "It is not the class of license the amateur holds, but the class of the 
> amateur that holds the
> license."
>
> Luc Leblanc VE2DWE
> AMSAT 33583
> Skype VE2DWE
> www.qsl.net/ve2dwe
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur satellite program!
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----
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